The present invention relates to a method of operating a refrigerant circuit as a heat pump as well as to a refrigerant circuit operable as a heat pump.
In the case of electric and hybrid vehicles, refrigerant circuits operable as heat pumps are increasingly considered for the temperature control of the passenger compartment and/or of individual vehicle components. At low outside temperatures (for example, temperatures below 0° C.), heat pump circuits with the component sizes customary in the case of conventional air-conditioning systems, i.e. the customary evaporator or condenser sizes, provide only a comparatively low heating power, which is less than the requirement of current vehicles. Heat pump concepts for motor vehicles known so far, as a rule, therefore have additional electric heating devices which allow an electric “auxiliary heating” at low outside temperatures.
It is an object of the invention to provide a method of operating a refrigerant circuit which allows a sufficiently high heating performance even at low outside temperatures, as well as providing a correspondingly suited refrigerant circuit.
This and other objects are achieved by a method of operating a refrigerant circuit as a heat pump, as well as a corresponding refrigerant circuit, wherein: refrigerant is compressed via a compressor and is pumped through a refrigerant/heating heat exchanger device; refrigerant coming from the refrigerant/heating heat exchanger device is expanded in at least a first expansion element; refrigerant expanded by way of the first expansion element flows to a suction input of the compressor through at least one evaporator; and, at a branch-off point of the refrigerant/heating heat exchanger device situated between a refrigerant input and a refrigerant output of the refrigerant/heating heat exchanger device, refrigerant is branched off and is guided by way of a second expansion element in the direction of the suction input.
One aspect of the invention is to ensure, by means of the development of the refrigerant circuit and by means of a suitable operating strategy, that the mass flow of the refrigerant taken in and compressed by the refrigerant compressor or the density of the refrigerant delivered by the refrigerant compressor is as high as possible (particularly, at low outside temperatures).
In principle, it could be considered to control the refrigerant volume flow by way of a rotational speed adaptation of the refrigerant compressor. However, the mass flow of the refrigerant not only depends on the refrigerant volume flow of the compressor but also on the density of the refrigerant at the inlet of the refrigerant compressor. In addition, the refrigerant flow cannot be arbitrarily increased because otherwise the temperature at the one or more evaporators of the refrigerant circuit would decrease excessively.
Both problems can be solved by way of the present invention.
A refrigerant circuit according to the invention has a refrigerant compressor which takes in refrigerant, compresses it and pumps it through a “refrigerant/heating heat exchanger device” or additionally a condenser/gas cooler in the air conditioner. The “refrigerant/heating heat exchanger device” may, for example, be a “refrigerant/fluid heat exchanger device” by way of which the heat is transferred from the refrigerant to a fluid transferred by pumping in a heating circuit, or a “refrigerant/air heat exchanger device” by way of which heat is transferred from the refrigerant (without an interconnected heating circuit through which fluid is flowing) to the air flowing into a passenger compartment.
Refrigerant coming from the refrigerant/heating heat exchanger device is expanded in at least a first expansion element. The expanded refrigerant flows through at least one evaporator of the refrigerant circuit and flows back from there in the direction of a suction input of the compressor.
The at least one evaporator may, for example, be an ambient heat exchanger which receives heat from the ambient air. In particular, several evaporators may also be provided, for example, an evaporator which cools a vehicle component, wherein the component is a high-voltage accumulator or the like.
According to the invention, at a branch-off point of the refrigerant/heating heat exchanger device situated between a refrigerant input and a refrigerant output of the refrigerant/heating heat exchanger device, refrigerant is branched off and is guided by way of a second expansion element in the direction of the suction input. The refrigerant/heating heat exchanger device may be a refrigerant/fluid heat exchanger device which cools and possibly also condenses the refrigerant, the transmitted heat being used for heating the interior by way of a heating circuit having a heating heat transfer device in the air conditioner. As an alternative, the refrigerant/heating heat exchanger device may be a refrigerant/air heat exchanger device which cools and possibly condenses the refrigerant, this heat, for heating the interior being utilized directly in the air conditioner. Depending on the refrigerant, only a cooling of the refrigerant or additionally also a condensation takes place in the refrigerant/heating heat exchanger device.
A refrigerant mass flow, which is branched off from the branch-off point of the refrigerant/heating heat exchanger device, can therefore be admixed to the refrigerant mass flow coming from the at least one evaporator. As a result, the density of the refrigerant taken in by the refrigerant compressor or the refrigerant mass flow can be set or controlled such that a heating power is obtained that is as high as possible also at low outside temperatures.
According to a further aspect of the invention, a second expansion element is provided in the section of the refrigerant circuit which is situated between the branch-off point of the refrigerant/heating heat exchanger device and the suction input of the refrigerant compressor.
The refrigerant/heating heat exchanger device can be thought of as being subdivided into a “first section”, which extends from the refrigerant input to the branch-off point. In this first section, the refrigerant, which is compressed by the refrigerant compressor and heated, is cooled, in which case it may be provided that it remains gaseous, i.e. superheated, on the entire first section. Correspondingly, the first section of the refrigerant/heating heat exchanger device could be called a “superdeheater”, by way of which gaseous refrigerant is cooled.
A “second section” of the refrigerant/heating heat exchanger device is formed by the section situated between the branch-off point and the refrigerant output. On this section, the refrigerant continues to release heat and condenses depending on the refrigerant or continues to be cooled supercritically. The second section could be called a “condenser or gas cooler”, in that the gaseous refrigerant coming from the first section is completely or partly condensed or is cooled supercritically.
According to a further aspect of the invention, the refrigerant circuit is thermally coupled by way of the refrigerant/heating heat exchanger device (in this context, refrigerant/fluid heat exchanger device) with a heating circuit, through which a fluid is flowing. The refrigerant/fluid heat exchanger device may be a heat exchanger device through which refrigerant and fluid flow in a counter-flow (counter-flow heat exchanger). The above-mentioned superdeheater section and the condenser- or gas cooler section may be formed by two sections of a single counter-flow heat exchanger. As an alternative, the superdeheater and the condenser or gas cooler may also be formed by two separate series-connected counter-flow heat exchangers.
The heating circuit has a refrigerant or water pump, the two sections (superdeheater section and condenser or gas cooler section) of the refrigerant/fluid heat exchanger device, and a heating heat exchanger. The heating heat exchanger, which is normally situated in the air conditioner, is a fluid/air heat exchanger by way of which heat from the fluid or the water can be released to the air flowing into a passenger compartment of a vehicle.
According to another aspect of the invention, the refrigerant/heating heat exchanger device is constructed as a refrigerant/air heat exchanger device. In this case, the refrigerant circuit releases heating heat by way of the refrigerant/air heat exchanger device, which is normally situated in the air conditioner, directly to the air flowing into a passenger compartment of a vehicle. In this case, the two parts of the refrigerant/air heat exchanger device may be connected in series on the air side, preferably the flow through the first part takes place after the flow through the second part on the air side.
According to a further aspect of the invention, following the refrigerant output of the refrigerant/heating heat exchanger device, i.e. between the refrigerant/heating heat exchanger device and the at least one first expansion element, a further branch-off point is provided, which, by way of a third expansion element, is in a fluid connection with the suction input of the refrigerant compressor.
The second and/or the third expansion elements may preferably be automatically controllable expansion elements. As an alternative, they may also be single expansion elements with a fixed flow cross-section, which can each be opened or closed.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.